1. Field of the Invention
The invention relates to an improved electromechanical wheel brake actuating device.
2. Description of the Prior Art
A wheel brake device of the type with which this invention is concerned has been disclosed by WO 96/03301. The known wheel brake device has an electric motor with an annular rotor, which can rotatingly drive a nut of a rotation/translation conversion gear embodied as a helical gear. The rotation/translation conversion gear converts the rotatory drive motion of the electric motor into a translatory motion so that a frictional brake lining of the wheel brake device can be pressed against a rotating brake body in order to generate a braking force or a braking moment. In order to release the wheel brake device, the frictional brake lining can be lifted up from the brake body by rotating the electric motor in the opposite direction. The known wheel brake device is embodied as a disk brake, the brake body is a brake disk non-rotatably connected to a vehicle wheel. In principle, the wheel brake device can also be used for other kinds of brake such as a drum brake.
In the wheel brake device according to the invention, the electric motor is embodied as a so-called transverse flux motor with an annular rotor. In contrast to conventional electric motors, which have a separate excitation coil for each pole, the transverse flux motor according to the invention has only one excitation winding per phase winding. The excitation winding is annular and encompasses an imaginary motor axis. In order to produce magnetic poles, the stator has yokes, which are distributed over the circumference of the excitation winding and can be magnetized by supplying current to the excitation winding. The yokes are preferably distributed equidistantly over the circumference, but this is not absolutely necessary. The supply of current to the excitation winding produces magnetic fields in the yokes. The annular excitation winding with the yokes distributed over its circumference will be referred to below as the excitation device of the transverse flux motor according to the invention. The excitation device preferably constitutes a stator of the transverse flux motor since this makes it easier to supply current.
In addition, the transverse flux motor of the wheel brake device according to the invention has a number of poles, which preferably corresponds to the number of yokes. The poles can be moved together in relation to the excitation device, on a circular path in the circumference direction of the yokes. If the excitation device constitutes the stator of the transverse flux motor, then the poles constitute its rotor, i.e. they are supported so that they can rotate together on a circular path around the imaginary motor axis. In order to produce a rotary motion, current is supplied to the excitation winding, i.e. the yokes are magnetized and magnetically attract the poles. The poles are pulled toward the yokes until the poles and yokes are aligned with one another. In order to produce the rotating motion, the excitation winding is supplied with current when the poles are offset in the circumference direction from the yokes. The poles move toward the yokes, i.e. the rotor rotates, until the poles and yokes are aligned with one another. Then the supply of current to the excitation winding is interrupted. In order to produce a more uniform concentric running of the transverse flux motor and a torque in every rotation position of the rotor, the transverse flux motor is preferably provided with three or more phase windings; each phase winding has an excitation device and associated poles (claim 3). This also assures that the transverse flux motor can start in every rotation position of its rotor and can start in the desired rotation direction. The supply of current to the excitation windings of the transverse flux motor of the electromechanical wheel brake device according to the invention is controlled electronically. In an embodiment of the transverse flux motor with permanent magnets, two phase windings are sufficient (claim 4). Such an embodiment of the invention has the advantage of a higher power density.
The wheel brake device according to the invention has the advantage that its electric motor requires only one excitation winding per phase winding instead of the usual one coil per pole required in conventional electric motors. Since the winding of coils and their attachment to the poles and yokes is expensive, this reduces the effort and cost involved in manufacturing. Another advantage of the wheel brake device according to the invention is that permanent magnets for its transverse flux motor can be eliminated, which can further reduce the effort and cost involved in manufacturing. Another advantage of the transverse flux motor is increased motor dynamics since it can easily be manufactured with a large number of poles, depending on how it is designed. An increase in the number of poles does not change the number of windings/coils. An increase in the number of poles therefore does not increase the manufacturing costs or only increases them slightly; the problem of not being able to accommodate or mount a large number of coils does not occur. Other advantages include an improvement in the efficiency, a volume reduction, and an increase in the power density. In addition, depending on how it is designed, the transverse flux motor is suitable for an annular design, as a result of which the rotation/translation conversion gear and a reduction gear possibly connected between the electric motor and the rotation/translation conversion gear can be accommodated in a cavity inside the annular transverse flux motor. In addition, situating the electric motor in an annular fashion around the gear produces a large lever arm and therefore a high driving torque of the electric motor.